The present invention relates to an output circuit which is provided between the output of a first circuit and the input of a second circuit formed in an integrated circuit such as a semiconductor integrated circuit, and which outputs a signal of a level based on a signal supplied from the first circuit to the second circuit. Such an output circuit is usually provided in an integrated circuit. In this case, the second circuit is referred to as an external circuit or an external interface.
FIGS. 9 through 11 are circuit diagrams illustrating conventional output circuits employed for MOS integrated circuits. FIG. 9 shows a CMOS output circuit in which an output section is configured using a CMOS circuit composed of a pMOS transistor 103 and an nMOS transistor 104. FIG. 10 shows an nMOS open-drain type output circuit which employs the nMOS transistor 104 having an open drain electrode. FIG. 11 shows an nMOS pull-up drain type (nMOS open-drain type with a built-in pull-up device) output circuit in which an output section is composed of a pMOS transistor 302, which stays ON constantly, and an nMOS transistor 104 having the drain electrode thereof pulled up by the pMOS transistor 302. There are also available a pMOS open-drain type output circuit and a pMOS pull-down drain type (pMOS open-drain type with a built-in pull-up device) output circuit. The nMOS pull-up drain type and the pMOS pull-down drain type are referred to simply as pull-drain type. In FIG. 9 through FIG. 11, an input signal IN is applied to an input terminal "in" and transmitted to an input signal conductor 10 via an inverter 101. An output signal OUT is output through an output terminal "out."
The conventional output circuits set forth above are provided with signal output types, namely, CMOS type, open-drain type, and pull-drain type, and a driving capability to comply with the specifications of an external interface to be connected thereto. This has been posing a problem in that, when the external interface connected thereto is changed, the output circuit that has been used before the change cannot be used as it is, meaning that the internal circuitry of the output circuit has to be changed. There has been another problem in that, when two or more external interfaces are expected to be used, the same number of output circuits compatible with the respective external interfaces as that of two or more external interfaces must be prepared in advance.